The present invention relates to a humidity detector using a humidity detecting element having a humidity sensing portion formed by sintering a metal oxide or metal oxides on a heat resistant wire coil.
There is well known a method for measuring the humidity in air, which utilizes the principle that the heat conductivity in a space varies depending upon the quantity of water vapor contained in the space, for example, as disclosed in U.S. Pat. No. 1,855,774.
The above method is realized by the construction made by the incorporation, in a bridge circuit, of a detecting element disposed in a measuring atmosphere in many cases and a reference element disposed in a space of the known humidity and having the same temperature-resistance characteristic as the detecting element.
A thermistor and a platinum wire are utilized as a detecting element.
A method as described in U.S. Pat. No. 4,419,888 is based on the aforementioned principle. This method involves supplying an electric current to a detecting element disposed in a measuring atmosphere and having a temperature characteristics found in a thermistor or the like to heat the detecting element to a temperature higher than that of such atmosphere. The resistance value of the detecting element is varied depending upon the quantity of water vapor contained in the above atmosphere, and this variation is detected. Then, the humidity in such atmosphere is detected from such variation.
There is also known a solid state thermal conductive gas detecting method which employs an element made by applying an n-type semiconductor such as SnO.sub.2 and ZnO onto a platinum wire, as disclosed in Japanese Patent Publication No. 34640/79. In such a detecting device, when an electron donative gas such as a combustible gas has been adsorbed on a gas detecting portion made of a metal oxide semiconductor such as SnO.sub.2, the electron concentration in the detecting portion increases, and with the increase in electric conductivity, the increase in electron concentration promotes an increase in heat conductivity. As a result, the two actions reducing the temperature of the detecting portion cause the resistance of the platinum wire coil disposed at the center of the detecting portion to be reduced, thus detecting the concentration of the gas which is to be detected.
However, the conventional devices are accompanied by various problems. The output of the heat conduction type humidity detecting element with platinum employed as a detecting element is substantially smaller than that of the aforementioned thermistor heat conduction type humidity detecting element. Moreover, the platinum coil detecting element is very sensitive in thermal dissipation at an operating temperature of about 200.degree. C. and is liable to be influenced by a slight mechanical vibration and wind.
In general, if an electric current is previously supplied so that the temperature of the detecting element may reach a level higher than the atmosphere temperature, the sensitivity can be improved. However, there is a problem that when the platinum coil detecting element is brought into a higher operating temperature, the output is unstable.
In the thermistor heat conduction type himidity detection, when the thermistor is heated to 200.degree. C. or more, it may be broken by the self-heating.
The element made by applying an n-type semiconductor on the platinum wire has a relatively high sensitivity because of the purpose of detecting a gas, but has a lower sensitivity in humidity and hence, is not suitable for use as a humidity detecting element. Further, because the temperature at which the metal oxide semiconductor has been sintered is relatively low (at 800.degree. C. to 900.degree. C.), the operation in a hot and humid atmosphere (e.g., at 80.degree. C. and 95% RH) causes the generation of a micro-crack which will grow into a large crack in a short time, resulting in a substantial variation in the resistance value. This is also a serious disadvantage.